1. Field of the Invention
The present invention relates to a surface acoustic wave device having an electrode structure with high withstand power at high frequencies.
2. Description of the Related Art
Surface acoustic wave devices are electronic components using surface acoustic waves, which are mechanical vibrations propagating along surfaces of solid materials, and they are used for filters, resonators, or duplexers.
Recently, the miniaturization of mobile communication terminals such as cellular phones has been advancing. Therefore, electronic components of such terminals must be reduced in size.
A surface acoustic wave device has a piezoelectric substrate and a pair of interdigital electrodes (interdigital transducer (IDT) electrodes), disposed thereon, made of a conductive material. Each interdigital electrode has electrode fingers that are alternatively arranged. The surface acoustic wave device having such a simple structure is suitable for filters, resonators, and duplexers that must be reduced in size when used for such mobile communication terminals.
A known surface acoustic wave device usually includes interdigital electrodes made of aluminum (Al) or an aluminum alloy having high conductivity and low density.
When such a surface acoustic wave device is used as, for example, an antenna duplexer placed in a radio frequency (RF) section which is placed downstream a transmitting amplifier and to which high voltage is applied, the surface acoustic wave device must have high withstand power. Further, since the mobile communication terminals have recently used higher frequencies, the operating frequency of the surface acoustic wave device must be increased from several hundred MHz to several GHz.
In order to increase the frequency, the electrode fingers of the interdigital electrodes of the surface acoustic wave device must be reduced in size and pitch between the electrode fingers. For example, band pass filters with a center frequency of 2 GHz must have a width of about 0.5 μm and band pass filters with a center frequency of 10 GHz must have a width of about 0.1 μm.
When high-voltage signals are applied to the interdigital electrodes having such fine electrode fingers, the interdigital electrodes suffer from strong stress due to surface acoustic waves. Stress exceeding the critical stress of the interdigital electrodes causes electromigration or stress migration. The electromigration or stress migration carries metal atoms contained in the interdigital electrodes to migrate through grain boundaries or stable faces of crystals and thereby voids are formed in the interdigital electrodes or hillocks are formed thereon. The electromigration or stress migration causes a breakage of the electrodes, electrical breaks, an increase in insertion loss of elements, a decrease in Q-factor of resonators, and the like, and finally causes deterioration of the surface acoustic wave device.
The known surface acoustic wave devices, disclosed in Japanese Unexamined Patent Application Publication No. 2002-305425 and U.S. Pat. No. 6,630,767, have an Al electrode layer having a twin crystal structure in which a base layer made titanium (Ti) is formed below an interdigital electrode section made of Al or an Al alloy, thereby enhancing withstand power.
When the base layer made of Ti is formed below the interdigital electrode section made of Al or the Al alloy, the orientation in a specific direction of the {111} plane of the Al layer or Al alloy layer is weakened and the electromigration or stress migration is apt to occur in the surface acoustic wave device. This causes a breakage of the electrodes, electrical breaks, an increase in insertion loss of elements, a decrease in Q-factor of resonators, and the like, and finally causes deterioration of the surface acoustic wave device.
Further, oxygen atoms of the piezoelectric substrate diffuse into the base Ti layer, or the Al layer or Al alloy layer. Therefore, there is a problem in that the electrical resistance of the interdigital electrode section becomes large, thereby increasing the insertion loss of elements.
U.S. Pat. No. 6,297,580 discloses a technique in which a layer made of TiN is formed below a layer made of an Al alloy. However, in this prior art, an example of a surface acoustic wave device, in which the layer made of TiN is formed below the layer made of the Al alloy, and the rules for liquid crystal orientation are not disclosed.